1. Field of the Invention
This invention relates to a process for continuously producing a propylene-ethylene block copolymer. More particularly it relates to a process for producing a propylene-ethylene block copolymer having very well balanced qualities such as high-impact properties, stiffness, processability, etc., with a high yield.
2. Description of the Related Art
Crystalline polypropylene produced using a stereoregular catalyst has raised a problem that it has superior characteristics in stiffness, heat resistance, etc., whereas it has a low high-impact strength, particularly that at low temperatures; hence its practical use application field has been restricted. Thus as a process for overcoming such a drawback, a number of processes for block-copolymerizing propylene with ethylene or other .alpha.-olefins have been proposed. Such block copolymerization processes make it possible to improve the low temperature high-impact strength of polypropylene to a large extent without reducing stiffness, heat resistance, etc. as superior characteristics of polypropylene so much, whereas problems in the aspects of production and qualities, specific of block copolymerization processes have been raised. Namely, in the case where the block copolymer is produced according to batch polymerization process, the yield of the copolymer per unit time, per unit polymerization vessel is lower than that in the case of continuous polymerization process, that is, the cost becomes higher. On the other hand, in the case of a multi-stage continuous polymerization process, there occurs a distribution (which is considered as being close to a complete mixing vessel distribution) relative to the retention times of the respective catalyst particles in the polymerization vessels of the respective steps so that there are formed aggregates of polymer particles having a distribution relative to the content proportions of the polypropylene part (a part containing a large quantity of propylene) and the polyethylene part (a part containing a relatively large quantity of ethylene); hence there is raised a drawback in the aspect of qualities originating from the non-uniformity of the distribution. In order to overcome such a drawback of the continuous polymerization process, a number of processes have been proposed. For example, according to Japanese patent application laid-open Nos. Sho 58- 49716/1983, Sho 55-116716/1980, Sho 58-69215/1983, etc., a process has been proposed wherein a slurry having left the propylene polymerization part is classified by means of a cyclone and the resulting fine particles are again returned to the propylene polymerization part. However, since the classification of the catalyst particle size does not always correspond to the retention time distribution, improvement in the non-uniformity is insufficient.
Japanese patent application laid-open Nos. Sho 57-195718/1982, Sho 58-29811/1983, etc. disclose a process wherein catalyst feed and slurry withdrawal from the polymerization vessel are intermittently carried out to reduce the quantity of catalyst entering the ethylene polymerization part within a short retention time, but a problem that the polymerization reaction is unstable has been raised.
Further, certain processes like that of the present invention have been proposed wherein the slurry having left the propylene polymerization part is treated with an electron-donative compound or the like to thereby selectively inactivate the catalyst particles having left the polymerization vessel within an insufficiently short retention time (short pass catalyst). For example, Japanese patent application laid-open Nos. Sho 58-32615/1983, Sho 57-174310/1982, Sho 57-174311/1982, Sho 57-147508/1982, etc. propose halogen compounds as an additive for the inactivation, but the effectiveness of selectively inactivating the catalyst particles is not yet sufficient. Still further, Japanese patent application laid-open Nos. Sho 57-145115/1982 and Sho 55-115417/1980 propose various electron-donative compounds, but even use of compounds in the range wherein they are used in the Examples has been still insufficient in the effectiveness for achieving the object of the present invention, i.e. a process for continuously producing a block copolymer having physical properties corresponding to those according to batch-wise polymerization process.